Surfgrass (Phyllospadix torreyi) reproduction: reproductive phenology, resource allocation and male rarity

Little is known about the reproductive ecology of aquatic angiosperms, despite their wide distribution and ecological importance. Surfgrass (Phyllospadix torreyi) is a clonal marine angiosperm that lives on turbulent rocky shores. Surfgrass exhibits a strongly female-biased sex ratio in flowering shoots throughout its distribution along the Pacific coast of North America. The ecological causes for this biased sex ratio were examined over 3 yr in a surfgrass population off Catalina Island in southern California, USA. Abundances, biomass, and C:N content of male and female flowering ramets were measured, and the reproductive phenology, reproductive losses to abortions, herbivore damage to fruits and seeds, and clonal survival and growth were quantified. Each year, a quarter of the vegetative ramets reproduced sexually and seed production in the population was prolific. The population sex ratio was female biased across the entire depth distribution of surfgrass, but males were more abundant deeper in the bed. Despite male rarity, pollen was abundant in the population, and virtually all ovules were fertilized, with few losses to abortions or herbivory before seed release. Potential female fitness (seeds produced) was highest at shallow depths where light availability was greatest and abortions were reduced. The spatial segregation of the sexes was not due to environmental sex lability but to greater ramification of female vs. male clones in shallow depths. Female flowering shoots had significantly higher biomass than males across water depth, despite no obvious cost in terms of vegetative growth, or advantage in terms of plant size. If current theory on sex allocation that the sex ratio should be biased toward the gender with the lower reproductive cost is true for surfgrass, then surfgrass provides an example of how a subtle resource allocation difference can result in a dramatic population bias. An alternative explanation for the female bias in surfgrass populations is greater male mortality due to significantly weaker attachment to the substratum. This study demonstrates the potential role hydrodynamics can play in aquatic plant reproduction and in structuring their populations.
Key words: biomechanics; Phyllospadix; seagrass reproduction; sex ratio; surfgrass. INTRODUCTION Dioecious plants with sex chromosomes are predicted to have primary sex ratios (i.e., in seeds at the end of parental [...]